Recently, in the field of image forming technology for copiers and printers, image formation to realize dot image reproduction at a level of 1200 dpi (dpi; number of dots per inch or 2.54 cm) has become feasible with progress of digital technology. It is effective to employ a semiconductor laser as a light source for exposure to perform such fine-dot image formation. Semiconductor lasers can be made to shorten their lasing wavelengths, which is effective to form a latent image at a smaller spot diameter. Recently, development of light-emitting diode technology has enabled realization of a laser at an emission wavelength as short as 350 to 500 nm Thus, shortening the wavelength of the light source for exposure has been promoted with the progress of semiconductor laser technologies, enabling to form images at a further smaller dot level and accelerating enhancement of resolution of electrophotographic images.
Meanwhile, designing a photoreceptor suitable for exposure light at such short wavelengths as described above is necessary to realize high resolution electrophotographic images and accordingly, it is an essential point to choose a compound, called a charge generation material to generate an electric charge on the photoreceptor upon exposure to light. There have been studied so far charge generation materials suitable for a short wavelength laser light. Specifically, the use of an α-type titanyl phthalocyanine as a charge generation material achieved a photoreceptor suitable for a laser light at an emission wavelength of 400 to 500 nm, as set forth in, for example, JP-A No. 9-240051 (hereinafter, the term “JP-A” refers to Japanese Patent Application Publication) but a satisfactory function has not come into effect for a light near 400 nm.
Consequently, there have been studied photoreceptors exhibiting enhanced sensitivity at an emission wavelength of not more than 400 nm For instance, an electrophotographic photoreceptor exhibiting a sensitivity to a laser light of 380 to 500 nm was developed by use of polycyclic quinone compounds or perylene compounds having a specific structure, as set forth in, for example, JP-A No. 2000-47408.